1. Field of the Invention
The present invention relates to a method of manufacturing a flexible wiring board, and in particular, to an improvement of a wiring layer included in a flexible wiring board frequently used for a multi-layered wiring board.
2. Description of the Related Art
Conventionally, a flexible wiring board having such a structure that a desired circuit pattern is formed on a surface of an insulating film is frequently used for various devices.
Since even a build-up of a plurality of such flexible wiring boards does not have a large thickness, the flexible wiring board is frequently used particularly for constituting a multi-layered wiring board by building up a plurality of wiring boards, each having a circuit pattern formed thereon.
A method of manufacturing such a multi-layered wiring board will be described below. FIGS. 66 to 75 are cross sectional views showing the steps of the manufacturing method.
First, a polyimide precursor is applied onto a surface of a metal foil such as a copper foil. The polyimide precursor is semi-cured to form a first base film made of polyimide. The first base film and the metal foil are bonded to each other in this state. Next, after a second metal foil is placed on the surface of the first base film, the second metal foil is heated while being pressed against the surface of the first base film to bond the first base film and the second metal film to each other. As a result, the first base film is sandwiched between two metal foils. Thereafter, a carrier film made of polyethylene terephthalate (PET) is provided onto the surface of one of the metal foils. This state is shown in FIG. 66. Reference numeral 111 in FIG. 66 denotes the first base film, and reference numerals 112 and 113 denote the metal foils, respectively. Reference numeral 114 denotes the carrier film.
Subsequently, the surface of the metal foil 113 is irradiated with laser light at a predetermined position for a plurality of times, wherein on the surface the carrier film 114 is not provided (hereinafter, referred to as a first surface-side conductive layer). As a result, the first surface-side conductive layer 113 and the first base film 111 are removed at the laser irradiated position so that a first via hole 115 is formed so as to penetrate the first surface-side conductive layer 113 and the first base film 111 to reach the metal foil 112 (hereinafter, referred to as a back-side conductive layer) on the side where the carrier film 114 is formed. This state is shown in FIG. 67.
Next, the first base film 111 including the first via hole 115 formed therethrough is immersed into an electroless plating solution (not shown) to grow an electroless plating layer 118 made of copper from the surface of the back-side conductive layer 112 over the surfaces of the first base film 111 which is exposed on the inner side face of the first via hole 115 and the first surface-side conductive layer 113. This state is shown in FIG. 68.
Subsequently, after the first base film 111 is immersed into an electrolytic plating solution containing copper (not shown), a DC voltage is applied between the electrolytic plating solution and the back-side conductive layer 112. As a result, a conductive material made of copper is grown on a surface of the electroless plating layer 118. Since the electroless plating layer 118 is provided over the entire surface of the first base film 111, the grown conductive material fills the first via hole 115 while covering the entire surface of the first base film 111. When the first via hole 115 is completely filled with the conductive material so that the surface of the conductive material becomes flat, the growth of the conductive material is terminated. Subsequently, a resist is applied onto the surface of the conductive material and is then patterned to form a resist film 190. This state is shown in FIG. 69, where reference numerals 116 and 190 denote the conductive material and the resist film, respectively.
Next, the conductive material 116, the electroless plating layer 118, and the first surface-side conductive layer 113 are etched by isotropic etching such as wet etching using the resist film 190 as a mask so as to pattern these layers into a desired pattern. As a result, a first surface wiring layer composed of the conductive material 116, the electroless plating layer 118 and the first surface-side conductive layer 113 is formed. This state is shown in FIG. 70, where reference numeral 120 denotes the first surface wiring layer.
At this moment, the conductive material 116 is grown to such a degree that the conductive material 116 fills the first via hole 115 to provide a flat surface thereof. Therefore, the conductive material 116 has a considerably large thickness. When the conductive material 116 having such a considerably large thickness is etched by isotropic etching such as wet etching to form the first surface wiring layer, there arises a problem in that a pattern width Δw1 of the actually formed first surface wiring layer 120 becomes smaller than a pattern width Δw0 of the resist film 190, that is, a desired pattern width, as shown in FIG. 70.
Next, after the resist film 190 is stripped off, a polyimide precursor solution is applied onto the surfaces of the first surface wiring layer 120 and the first surface-side conductive layer 113. Then, the polyimide precursor solution is semi-cured to form a second base film made of polyimide. This state is shown in FIG. 71, where reference numeral 151 denotes the second base film. The second base film 151, and the first surface wiring layer 120 and the first surface-side conductive layer 113 are bonded to each other in this state.
Next, after a third metal foil is placed on a surface of the second base film 151, the third metal foil is heated while being pressed against the surface of the second base film 151 to bond the second base film 151 and the third metal foil (hereinafter, referred to as a second surface-side conductive layer) to each other. Thereafter, the surface of the second surface-side conductive layer is irradiated with laser light for a plurality of times. As a result, a second via hole is formed at the laser irradiated position so as to penetrate the second surface-side conductive layer and the second base film 151 to reach the first surface-side wiring layer 120. This state is shown in FIG. 72, where reference numerals 153 and 155 denote the second surface-side conductive layer and the second via hole, respectively.
Subsequently, the first base film 111 is immersed into an electroless plating solution (not shown). Then, an electroless plating layer 158 made of copper is grown from the surface of the second surface-side conductive layer 153 over the surfaces of the second base film 151 which is exposed on the inner side face of the second via hole 155, and the first surface wiring layer 120. The state after the growth of the electroless plating layer 158 is shown in FIG. 73.
Thereafter, as shown in FIG. 74, a conductive material 156 made of copper is grown on the entire surface of the electroless plating layer 158 by electrolytic plating. After the conductive material 156 is grown to such a degree that the conductive material 156 fills the second via hole 155 to provide a flat surface. Then, a second surface-side wiring layer 170 composed of the conductive material 156, the electroless plating layer 158, and the second surface conductive layer 153 is formed by patterning as shown in FIG. 75. Thereafter, the metal foil 112 on the back side is patterned to form a wiring layer 121 on the back side, thereby completing a double-layered flexible wiring board 101.
As described above, however, the conductive materials 116, 156 and the like, each having a considerably large thickness, are patterned by isotropic etching such as wet etching to form the first and second surface wiring layers 120 and 170. As a result, there arises a problem that a pattern width of each of the surface wiring layers 120 and 170 becomes smaller than a desired pattern width.
Moreover, an increased thickness of each of the surface wiring layers 120 and 170 also disadvantageously increases a thickness and weight of the flexible wiring board 101. Since these problems appear in a more notable manner with increase in the number of layers constituted by the flexible wiring board, such problems present major obstacles to increase in the number of layers.